We will study the effects of ionizing radiation at low dose rate on human diploid fibroblasts from patients with known or proposed DNA repair defects. The responses of these cells (ataxia telangiectasia, xeroderma pigmentosum and Fanconi's anemia) will be compared to the response of normal human fibroblasts. Radiation damage time interactions will be studied as these are expressed by survival, ie., capacity to divide without limit, and repair capacity. Two culture conditions will be used: 1) exponential and 2) plateau phase of growth.